Electrophotographic plate and method of producing same



Patented Dec. 22, 1953 ELECTROPHOTOGRAPHIC PLATE AND METHOD OF PRODUCINGSAME Arthur E. Middleton, Columbus, Ohio, assignor, by mesneassignments, to The Haloid Company, Rochester, N. Y., a corporation ofNew York No Drawing. Application May 25, 1949, Serial No. 95,374

13 Claims.

This invention relates to electrophotographic plates and to a method ofproducing such plates. More particularly, the invention relates to acomposition of matter including a photoconducting insulating materialand to an electrophotographic plate containing a coating of suchcomposition. Specifically, the invention relates to a method ofproducing a composition of matter containing a photoconductinginsulating material and to a method of making an electrophotographicplate cnated with such composition.

In the electrophotographicprocess, a base plate of low electricalresistance having aphotocom ductin insulatingcoating thereon iselectrostati cally charged in the dark. The charged coating is thenexposed to light through a negative photographic film, positive film ormask, or otherwise to a light image, to receive a latent electrostaticimage. If the plate hasbeen properly prepared,

the charges leak off rapidly to the base plate in proportion to theintensity of light to which any given area is exposed, and after suchexposure, powder is dusted on the coating in the dark and adheres to theareas where the electrostatic charges remain, forming a powder imagecorrespondin to the electrostatic image. The powder image can then betransferred to a sheet of transfer material resulting in a positive ornegative print as the case may be, having excellent detail and quality.

One object of the invention is to improve the a photoconductivecharacteristics or the plate and insure proper functioning of the platewhen exposed, this being accomplished by producing a composition ofmatter containing a photoconducting insulating material which can beapplied as a coating on a base plate in a practical and economicalmanner, and which adheres firmly thereto and acts successfully as aphotoconducting insulating layer.

Another purpose of the invention is to provide a method for coating abase plate at room temperatures with a composition containing aphotoconduoting insulating material, and which will adhere permanentlyand securely to the base plate and act efficiently as a photoconductininsulating layer.

A further object of the invention is to provide a composition of mattercontaining a ph'otoconducting insulating material and which will adherereadily and permanently to a base plate and act as a photoconductinginsulating layer.

An additional purpose of the invention is to provide an improved andmore efficient electrophotographic plate having on its surface anadherent coating comprising a, composition containing a photoconductinginsulating material, and capable of acting as a photoconductinginsulating layer.

It has now been found that an efiicient electrophotographic plate can beobtained by spraying a base plate at room temperature with aphotoconducting insulating composition prepared by intimately mixing andgrinding together a photoconducting insulating material, a highelectrical resistance binder, and a solvent. On evaporation of thesolvent, the composition forms a successful adherent layer on the platethat will hold electrostatic charges in the dark and rapidly lose themon exposure to light.

In general, the invention comprises intimately mixing and grindingtogether in a ball mill or other suitable fashion a photoconductinginsulati'ng material such as pure amorphous selenium with a solvent anda binder material having an electrical volume resistivity of about 10 toabout 10 ohms-cm, for a period of time sufficient to insure intimatemixture and reduction of the size of the particles. At the end of themilling period additional solvent may be added and stirred into themixture suificiently to render it sprayable. The resulting compositioncan readily be sprayed at room temperatures onto a clean, unheated baseplate. After the solvent has evaporated from the composition, thecoating can be electrostatically charged and used in theelectrophotographic process.

The selenium must be of the amorphous type and free of impurities suchas copper, iron, lead and bismuth which appear to aiTect adversely itsability to hold electrostatic charges by decreasing its electricalresistance so that the charges leak off rapidly in the dark and powderimages cannot be obtained. It is preferred to use amorphous A. R. Q.selenium (ammonia reduced in quartz from SeOz) in pellet form, fromabout 1 6-" to A size, as manufactured by the Canadian Copper RefinersCompan Limited, Montreal East, Ontario, Canada, and containing lessthanabout 20 parts per million of impurities.

Other grades and types of selenium as furnished commercially can be usedif purified and converted into the amorphous state. To obtainessentially purified amorphous selenium from the impure forms, it is theusual practice to first free it of the copper, iron, lead or bismuth bydistillation and then to melt down the required amount of selenium in acontainer exposed to the atmosphere by heating to a temperature of from250 C. to-300" C., and preferably about 250 C. The

low-boiling point hydrocarbon solvent, and like the binder should notintroduce impurities which would lower the electrical resistance of thecoating. The solvent should preferably be one that is suitable for theparticular binder empl since the solvent must not react with the bindermaterial to a 3; fierized product or similar substances which cannot besprayed. The solvent should not affect the adhesiveness of the binder orits electrical resistance, nor should it adversely affect the pureamorphous selenium. Part of the solvent is added to the selenium binderin the mill to provide good grinding viscosity and the balance may beadded after milling to increase the fluidity of resulting mixture. Thetotal amount of solv used is from about to about 3 parts by volume ofthe sol vent to about 1 part by volume of the binder and this proportionhas resulted in readily sprayable compositions. It is preferred,however, to use from about 0.8 to about 1.4: parts per volume of solventto about 1 part by volume of binder in the composition. Suitablesolvents for use with the binders disc osed herein are toluene,Cellosolve acetate, ;.y no, gasoline, fimsco F, benzene, methyliso-butyl get-one or mixtures thereof.

Cellosolve acetate is an organic chemical whose formula isCzHsOCHzCHzOOCCHs. Amsco F is an aromatic petroleum solvent manufacturedby American Mineral spirits Company, Chicago, 111., with the followingapproximate characteristics: specific gravity (60 F.) 0.8028, 77 percent aromatics, refractive index of 1.4905, mixed aniline point of 86F., flash point 130 F., and Kouri butanol value of 70. When less solventis present, it may be necessary to warm the composition slightly inorder to spray it. The temperature of the composition should, however,not exceed 50 C. in order to obviate the production of the hexagonalform of selenium.

The base material or plate should have an electrical resistance lessthan either the binder material or the amorphous selenium so that itwill act as a ground when the electrostatically charged coating isexposed to light. The base plate should be at room temperature and atleast not above about 50 (3., while being sprayed with thephotoconducting insulating composition, in order to prevent formation ofany crystals of hexagonal selenium in the coating which would reduce ordestroy its electrophotographic usefulness. Materials having electricalresistivities of less than 10 ohms-cm. are satisfactory as a plate forthe purposes of the invention, although it is preferred to use materialshaving an electrical resistivity of less than about 10 ohms-cm.

The base plate is generally flat, square or rectangular but can be ofany desired shape. The surface of the plate should be cleaned beforecoating with the selenium composition in order to remove grease andother dirt which might prevent firm adherence of the coating to the baseplate. Such cleaning can be accomplished by washing the plate with anysuitable alkali cleaner, or with a hydrocarbon solvent, followed byrinsing. Any gross surface irregularities should be removed by grindingor polishing, although it is not necessary to polish the plate until ithas mirror-like reflectivity. Acceptable materials for the base platehave been found to be aluminum,

brass, glass, aluminum-coated glass, stainless steel, nickel, steel,bronze, copper, engravers copper, engravers zinc, and grainedlithographic zinc, although other materials having electricalresistances similar to the aforementioned can also be used as baseplates to receive the composition coated thereon.

It is not necessary to add the selenium to the binder in the mill in theform of finely divided particles, but it can be added in the form ofpellets of about 1 5" to diameter. The selenium, binder and solventshould be thoroughly mixed and ground up and as a result the selenium isreduced in size and thoroughly dispersed throughout the hinder or binderand solvent so that no surface irregularities or masses of unmixedparticles appear in the coating on low magnification. It is preferred tomix and grind, or mill, the mass until the particles in the compositionhave a size less than about 50 microns, although compositions havingparticles up to about .0002 inch in size have proven satisfactory.Instead of adding the amorphous selenium in pellet or pulverized form tothe mill, followed by separate additions of binder and solvent, asolution of the binder in the solvent can be added to the selenium inthe mill. It is only essential that enough solvent be present duringmilling to give good grinding viscosity.

Any commercially available mixing and grinding machinery can be utilizedfor this step, although it is desirable to use a ball mill, and at roomtemperature. The mill is not heated or cooled, although while the roomtemperature is about C., it is believed that the composition adjacentthe balls may have a temperature of about C. due to frictional heatdeveloped by the balls during grinding. Ceramic, glass, or steel ballscan be used in the mill. Steel balls, however, have been found best asthey produce a better mixing and grinding action. The time of mixing canvary from 3 to 72 hours, although it has been found preferable to mixfor about 10 to 20 hours to reduce the particles to approximatelymicrons in size. The mixing equipment should also be thoroughly cleanedbefore mixing to prevent the introduction of any impurities which mightadversely affect the resulting selenium composition.

At the end of the milling period, additional solvent is added to themixture and the mill operated for a short time to remove portions of themixture adhering to the walls and balls of the mill and to enable it tobe readily poured. This insures sufficient fluidity so that thecomposition can be easily sprayed, and provides a complete dispersion ofparticles in the solvent carrier or a homogenous solution or mixture.More than per cent recovery of materials is thereby made possible, andwhile the mixture at the end of the milling period actually contains adispersion of selenium in the binder and can be used to coat plates toprovide a photoconducting insulating layer thereon, is apparent thatadditional solvent is indicated as a means to facilitate spraying orcoating of the plates.

Commercially available paint-spraying equipment can be used to spray thecomposition onto the base plate which is at room temperature. The

. paint-spraying mechanism should be thoroughly cleaned prior to use toprevent introduction of harmful impurities into the coating mixture. Itshould also provide means for heating the composition when notsufliciently fluid. The equipment should be flexible and provided withadjusting means to enable the operator to readily produce on the baseplate coatings having thicknesses of from .0003 to .002 inch. Thickercoatings can be applied by longer spraying if desired, although thickercoatings increase the exposure 7 time:fartherelectrophotographic plate.If the coating is too thin, pinholes are apt to occur so that acontinuousfilrn is not produced. Thus, coatings as thin as .0003 inchrepresent about the practical lower'limitfor spraying. It will beobviousto those skilled in the art that other methods besides sprayingcan be utilized to produce coatings of the composition on the baseplate, as for example,. the composition can be applied by means of abrush or draw blade, by dipping, or by roller coating.

The following examples will serve to illustrate the invention with moreparticularity to those skilled. in the art:

Example 1 .200 grams of pelletized amorphous, black pure selenium, 4414grams of Parlon, 19.2 grams of Rezyl and '90 grams of toluene wereplaced in a quart (Bacilli') size steel ball mill filled with inch steelballs up to about the half-way mark and milled for about fourteen hours.After mixinc; about 120 grams of additional toluene was added to themixture to render it sprayable and the resulting red composition placedin a paint sprayer and sprayed onto a clean, fiat unheated aluminumplate. As soon as the plate had been coated with a thin film of thecomposition about .0005 inch thick, the spraying was stopped and thecoating allowed to dry by evaporation of the solvent. The hard, drycoating was completely adherent to the base plate. The plate was thencharged electrostatically and used in the electrophotog'raphic process,resulting in prints having excellent detail and no background.

Example 2 Component: Amount, gms.

Amorphous selenium 100 Vinylite VYHH 15.2 Vinylite VMCH 15.2 Methylisobutyl ketone 60 Toluene 60 The ingredientswere charged to a pint-sizeglass jar approximately half filled with inch glass balls and ground forapproximately 20 hours. After milling, the mixture was drained from thejar and thinned to spray viscosity with a 50:50 mixture of toluene andmethyl isobutyl ketone.

Example 3 Component: Amount, gms.

Vitreous selenium 100 Polystrene 23.8

The components were charged to a pint-size glass jar filledapprom'mately half full with inch glass balls, sumcient xylene beingadded to give grinding viscosity (50-100 seconds No. 4 Ford cup), andthen ground for approximately 20 hours. After milling the mixture wasdrained from the mill and thinned with xylene to spraying viscosity.

Example 4 Component: Amount, gms.

Vitreous selenium 1G0 Acryloid A10 89.2

The components were charged to a pint-size glass jar filledapproximately half full with /2 inch glass balls, thinned to suitablegrinding viscosity with Cellosolve acetate, and ground for '72 hours.The mixture was then drained from the mill and thinned to sprayingviscosity.

Example 5 Component: Amount, gms.

Vitreous seleniurnrn; o Acryloid B-72 31.3 Toluene 100 The selenium,acryloid and toluene were charged to a steel ball mill filledapproximately half full with /2 inch steel balls and ground for 24hours. The composition was then drained from the mill and thinned withadditional toluene to spraying viscosity.

Example 6 Component: Amount, gms.

Amorphous selenium Silicone (DC-804) 45.8

The ingredients were charged to a steel ball mill approximately halffilled with ,4 inch steel balls, toluene was then added to give suitablegrinding viscosity, and the composition was ground for 42 hours. Theresulting material was then drained from the mill and thinned withtoluene to spraying viscosity.

X-ray studies were made of electrophotographic plates containing thephotoconducting insulating coatings described herein, and revealed thatsome coatings appeared to contain only amorphous selenium in the binder,others apparently contained only alpha monoclinic selenium, while stillothers seemed to contain both amorphous and alpha monoclinic seleniumdispersed throughout the binder. All of the plates, however, producedexcellent powder images when used in the electrophotographic process.This appears to indicate that coatings containing the alpha monoclinicform of selenium, and those containing both the alpha monoclinic andamorphous forms of selenium are as effective as coatings containing onlyamorphous selenium as the photoconducting insulating material or activeingredient.

The presence of alpha monoclinic selenium in the coatings andcompositions is indicated from the fact that the black, amorphousselenium mixture turned red during milling, since alpha mono,- clinicselenium is red although this is not conclusive because very finelydivided amorphous selenium is also red. It is not thoroughly understoodwhy alpha monoclinic selenium appears in the composition after milling,although it would appear that the milling, or the other componentspresent during milling, or a combination of these factors, favorformation of the alpha monoclinic form. In any event, however,compositions or coatings containing only alpha monoclinic selenium asthe active material, or alpha monoclinic selenium in the presence ofamorphous selenium, seem to produce as efiective plates as thosecontaining essentially only amorphous selenium.

The invention teaches that electrophotographic plates can be readilyproduced by spraying a base plate with a composition prepared by millingamorphous selenium, a high electrical resistance binder, and a solvent,to which mixture has been added, after milling, additional solvent torender it sprayable. The finely divided particles of selenium appear tobe thoroughly dispersed throughout the high electrical resistance binderand are probably coated with the binder material or substantiallyinsulated from each other by the binder. The coated plate, orelectrophot-ographic plate, on drying can be charged electrostaticallyand will hold electrostatic charges in the dark and rapidly dissipatethem on exposure to light.

The invention is also applicable to the making of photocell andrectifier plates, and this application is intended to cover suchmodifications, departures, and adaptations as may come Within the scopeof the following claims.

I claim:

1. An electrophotographic plate in which the photoconducting insulatingcoating includes at least one material of the group consisting of 2. Anelectrophotographic plate comprising a base plate and an adherentcoating on one surface thereof, said coating comprising essentially amixture of amorphous selenium uniformly dispersed in a binder comprisinga continuous film of uniform thickness of thermoplastic synthetic resinbinding together the individual particles of selenium and holding thelayer on the plate, said binder having an electrical resistance higherthan that of the plate and selenium.

3, An electrophotographic plate comprising a base plate and an adherentcoating on one surface thereof, said coating including a mixture ofalpha monoclinic selenium uniformly dispersed in a binder comprising acontinuous film of uniform thickness of thermoplastic synthetic resinbinding together the individual particles of selenium and holding thelayer on the plate, said binder having an electrical resistance higherthan that of the plate and selenium.

4. An article of manufacture consisting of a base plate and an adherentphotoconducting insulating coating thereon, said coating including amixture of equal parts by volume of substantially pure amorphousselenium uniforml dispersed in a binder comprising a continuous film ofuniform 1.

thickness of thermoplastic synthetic resin binding together theindividual particles of selenium and holding the layer on the plate,said binder having an electrical resistivity of from 10 to 10 ohms-cm.

5. A coated plate, the coating thereof containing essentially finelydivided particles of amorphous selenium and alpha monoclinic seleniumdispersed throughout a continuous phase of a binder comprising acontinuous film of uniform thickness of thermoplastic synthetic resinbinding together the individual particles of selenium and holding thelayer onto the plate, said binder having an electrical resistivity offrom 10 to 10 ohms-cm.

6, An article of manufacture comprising a base plate having anelectrical resistivity less than about it) ohnis-cm. and a.photoconducting in sulating coating on the surface thereof, said coatingincluding a mixture of from 1 t0 2 parts by volume of a bindercomprising a continuous film of uniform thickness of thermoplasticsynthetic resin binding together the individual particles of seleniumand holding the layer onto the plate, said binder having an electricalresistivity of from to 10 ohms-cm. and 1 part by volume of at least onematerial of the group consisting of essentially pure amorphous seleniumand alpha monoclinic selenium.

7. A composition of matter useful in the production ofelectrophotographic plates, including a solution of an organic, easilyevaporated solvent, a binder selected from the group consisting ofpolystyrene, chlorinated rubber, silicone resin, acrylic resin, andvinyl-chloride-acetate resin, and at least one material of the groupconsisting of substantially pure amorphous selenium and alpha monoclinicselenium uniformly dispersed throughout said binder which securestogether "ill the individual particles of selenium and holds.

the layer on the plate, the binder being substantial in volume andhaving a resistance higher tllian that of said last named material andof the p ate.

8, A composition of matterincluding a binder comprising a thermoplasticsynthetic resin having an electrical resistivity of from 10 to 10ohms-cm, an organic, easily evaporated solvent for said binder, andfinely divided particles of amorphous selenium uniformly dispersedthroughout said binder.

9, A composition of matter including from 1 to 2 parts by volume of abinder comprising a thermoplastic synthetic resin having an electricalresistivity of from 10 to 10 ohms-cm, from 1.6 to 2.8 parts by volume ofan easily evaporated hydrocarbon solvent for said binder, and about 1part by volume of alpha monoclinic selenium of a particle size of from 1to 50 microns, said selenium being uniformly dispersed throughout saidbinder.

10. The method of producing a composition of matter which is insulatingand photoconducting and adherent after being applied as a coating to abase plate, which comprises intimately mixing and grinding in a ballmill for from 3 to '72 hours substantiall pure amorphous selenium, abinder comprising a thermoplastic synthetic resin having an electricalresistivity of from 10 to 10 ohms-cm, and an organic, easily evaporatedsolvent.

11. In the method of producing a composition containing seleniumcharacterized by being adherent, photoconducting and insulating afterbeing sprayed onto a base plate and drying, the step consisting ofmilling in a ball mill for from 10 to 20 hours one part by volume ofamorphous selenium containing not greater than about 20 parts permillion of impurities and from 1 t0 2 parts by volume of a bindercomprising a thermoplastic synthetic resin having an electricalresistance higher than that of the plate and selenium and selected fromthe group consisting of Parlon, polystyrene, silicone, acryloid, andvinylite copolymer, and an organic, easily evaporated solvent, the typeof mill specified and the conditions set forth being such as not topromote crystallization of the selenium and after said milling, addingan additional amount of said solvent to the resulting mixture to providea homogeneous mixture and to enable said mixture to be sprayed.

12 In the method of producing an electrophotographic plate, the stepscomprising milling in a ball mill for from 10 to 20 hours, essentiallypure amorphous selenium, an organic, easily evaporated solvent, and abinder comprising a thermoplastic synthetic resin having an electricalresistivity of from 10 to 10 ohms-0111., the type of mill specified andthe conditions set forth being such as not to promote crystallization ofthe selenium, mixing an additional amount of said solvent in theresulting mixture to provide a homogeneous mixture and to enable saidmixture to be sprayed, and then spraying the mixture containing thesolvent onto a base plate, having an electrical resistivity less thanabout 10 ohms-cm. to provide a photoconducting insulating layer thereon.

13, In the method of producing an electrophotographic plate, the stepcomprising spraying a base plate at room temperature with a compositionof matter including a binder comprising a thermoplastic synthetic resinhaving an electrical assess 11 resistivity of from 10' to 16 ohms-cm, avolatile Number hydrocarbon solvent, and at least one material of2,168,259 the group consisting of substantially pure amor- 2,178,018phous selenium and alpha. monoclinic selenium, 2,197,552

said. material having a particle size of from 1 to 5 2,199,104 50microns and being dispersed throughout said 2,297,691 binder. 2,433,401

Name 1 D Gorlich .i Aug. 1, 1939 Hofmann Oct. 31, 1939 Kuymick Apr. 16,1940 Johnson et a1 Apr. 30, 1940 Carlson Oct.- 6, 1942 Saslow Dec. 30,194! Korn et a1 Feb. 24, 1948 Gray Oct. 25, 1949 FOREIGN PATENTS CountryDate Germany fin-. 6. Sept. 18, 1942

1. AN ELECTROPHOTOGRAPHIC PLATE IN WHICH THE PHOTOCONDUCTING INSULATINGCOATING INCLUDES AT LEAST ONE MATERIAL OF THE GROUP CONSISTING OFAMORPHOUS SELENIUM AND ALPHA MONOCLINIC SELENIUM IN A BINDER COMPRISINGA CONTINUOUS FILM OF UNIFORM THICKNESS OF THERMOPLASTIC SYNTHETIC RESINWHICH AFFORDS AN EFFECTIVE MEDIUM WHICH UNIFORMLY DISPERSES SAIDSELENIUM AND OBTAINS GOOD ADHERENCE OF THE COATING TO THE PLATE, SAIDBINDER HAVING AN ELECTRICAL RESISTANCE HIGHER THAN THAT OF THE PLATE ANDPHOTOCONDUCTING INSULATING MATERIAL.